1. Field of the Invention
The present invention relates to a direct metal making method that utilizes microwave energy to reduce and melt metal-containing material, and separate molten metals from slag.
2. Description of the Related Art
Current processes for the extraction of metals from their respective ores are characterised by high energy consumption and the release of environmentally undesirable by-products, including large quantities of fine particulate, SO.sub.2, CO.sub.2, and NO.sub.x.
Historically, steel making has been practised using an indirect method whereby iron is first produced from ore or scrap metal and then converted into steel. In a typical iron smelting process, iron ore is ground to 500 mesh to liberate iron oxides from other minerals. The resultant material then goes through a separation process such as froth flotation to concentrate iron oxides. The resulting fine particles are normally pelletized with limestone and bentonite. Iron ore pelletizing and sintering are necessary to provide the required permeability for blast air and strength to support the heavy load in a blast furnace. The pellets, or masses, are sintered to make them strong, and charged, along with coke, into a furnace in which the pellets and coke are subjected to a blast of very hot air. In the furnace, the iron oxide is reduced and melted. During the melting, the iron picks up carbon and sulphur from the coke. To make steel, the carbon content of the iron must be reduced. This is commonly done in a basic oxygen furnace (BOF). Pure oxygen is blown, at supersonic velocity, into a molten mass, including liquid iron, contained in a bottle-shaped furnace. The oxygen reacts with the carbon in the molten iron to form CO and CO.sub.2. Sulphur, which is harmful to most steels, is removed by injecting a powdered material into the molten steel to form sulfides. The sulfides are collected as slag from the top of the molten mass.
This production route is very energy and material inefficient and causes serious environmental problems. The whole process is very dusty and noisy, and consequently poses health and environmental problems for workers and others in the area. Coke must be used to generate a temperature high enough for melting iron. Adding coke to the mixture, however, introduces carbon and sulphur into the iron. These elements must then be removed in subsequent processing. Also, the production of coke is an environmentally unfriendly process, and recently a shortage of coke has been a serious problem.
The production of other metals such as copper, nickel, lead, zinc and ferro-alloys present similar problems. SO.sub.2 emission is an additional problem for ores containing sulphur.
Various methods have been used to supply the heat necessary to melt metal and the material in which it is borne so that they may be separated. These include the burning of fossil fuels such as coal, coke, and oil, and the use of electric heaters.
Electric induction heating has been particularly useful because it introduces no additional contaminants into the metal being melted and produces no local emissions. One drawback of induction heating, however, is that it relies on the conduction of eddy currents within the material being heated. Induction heating is impossible if the material is not an electric conductor such as a metallic ore. Typically induction heating is only used where scrap metal is available in the initial charge to the furnace.
Electric arc heating is a popular method to produce metals from scraps. As with induction heating, the material to be heated must be an electric conductor. Metallic ores can't be heated directly by electric arc.
Microwave heating, as disclosed herewithin, transmits energy to electrically nonconductive materials or small agglomerations of metallic material more efficiently than induction heating or electric arc heating. It thus provides an alternative to the burning of fossil fuel, and can do the initial heating that makes later use of induction heating or electric arc heating feasible.
Various processes have been developed utilizing microwave energy in the purification of metallic compounds. U.S. Pat. No. 4,321,089 discloses a process for the recovery of molybdenum and rhenium from their sulfide ores. In the disclosed process, the sulfide ores are subjected to microwave energy in the presence of oxygen or chlorine to form oxides or chlorides respectively. In neither case is the metal reduced. These oxide or chloride intermediates are then subjected to additional processing under reducing conditions to produce metal. Both of these processes differ from the direct reduction processing disclosed herewithin, inasmuch as the microwave processing results only in an oxidized intermediate.
U.S. Pat. No. 4,324,582 (the '582 patent), also to Kruesi et al., discloses a process applying microwave energy to convert copper compounds into other compounds, such as oxides and chlorides, from which copper is more readily recoverable. The claims of the '582 patent are restricted in scope to using microwave energy "to convert the sulfidic and oxidic compounds in the ores to compounds from which copper is more readily recoverable".
The specification of the '582 patent specifically teaches away from ferrous metal processing as disclosed herein, asserting that "the oxides of iron and chromium, which are transition metals, do not absorb microwaves," and "the gangue of the ore does not appreciably absorb microwave radiation".
In contrast to the preparation of an intermediate material as disclosed above, the process of the present invention results directly in a purified metal by the chemical reduction of oxides, sulfides, and other ores and metal sources through the application of microwaves and appropriate reducing agents in combination with induction heating or electric arc heating.
U.S. Pat. No. 5,131,941 (the '941 patent) to Lemelson issued Jul. 21, 1992, discloses a process for refining metal from ore, including flowing a stream of small particles of ore to a reaction zone. The process disclosed in the '941 patent is very different from that of the instant disclosure. The '941 patent process is adapted to the processing of fine particles in a flowing stream without the benefit of reducing agents. It is fundamentally directed to beam energy processes, unlike the present invention which is directed to the processing of massed material in a microwave cavity.
It is clear that the above-mentioned examples from the prior art do not possess the novel attributes of the present invention, namely the clean direct production of metals and efficient use of materials for the metal industry. This invention presents a revolutionary method to produce metals directly from ores by utilizing microwave energy as the primary heating source. This process is dramatically different from any of the current metal making techniques. The foreseeable advantages of this new metal making method over the traditional metal making methods include reductions in energy consumption and combustion emissions, the elimination, or reduction, of coke with its related environment problems, lower capital investment, and lower production cost, and minimal contamination of the metal produced.